High power density super-conducting electric machine

ABSTRACT

A high power density synchronous machine is disclosed comprising: a stator having conventional stator coils arranged in an annulus around a vacuum cylindrical cavity; a magnetically saturated cylindrical magnetic solid rotor core; a race-track super-conducting coil winding extending around the rotor core, and a coil supportextending through the core and attaching to opposite long sides of the coil winding.

RELATED APPLICATIONS

This application is related to the following commonly-owned andcommonly-filed applications (the specifications and drawings of each areincorporated herein):

U.S. patent application Ser. No. 09/854,932 entitled “SuperconductingSynchronous Machine Having Rotor And A Plurality Of Super-ConductingField Coil Windings”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,933 entitled “High TemperatureSuper-Conducting Rotor Coil Support With Split Coil Housing And AssemblyMethod”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,931 entitled “SynchronousMachine Having Cryogenic Gas Transfer Coupling To Rotor WithSuper-Conducting Coils”, filed May 15, 2001;

U.S. patent application Ser. No. 09/855,026 entitled “High TemperatureSuper-Conducting Synchronous Rotor Coil Support With Tension Rods AndMethod For Assembly Of Coil Support”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,946 entitled “High TemperatureSuper-Conducting Rotor Coil Support With Tension Rods And Bolts AndAssembly Method”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,939 entitled “High TemperatureSuper-Conducting Coils Supported By An Iron Core Rotor”, filed May 15,2001;

U.S. patent application Ser. No. 09/854,938 entitled “High TemperatureSuper-Conducting Synchronous Rotor Having An Electromagnetic Shield AndMethod For Assembly”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,940 entitled “High TemperatureSuper-Conducting Rotor Coil Support Arid Coil Support Method”, filed May15, 2001;

U.S. patent application Ser. No. 09/854,937 entitled “High TemperatureSuper-Conducting Rotor Having A Vacuum Vessel And Electromagnetic ShieldAnd Method For Assembly”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,943 entitled “Cryogenic CoolingSystem For Rotor Having A High Temperature Super-Conducting FieldWinding”, filed May 15, 2001;

U.S. patent application Ser. No. 09/854,464 entitled “High TemperatureSuper-Conducting Racetrack Coil”, filed May 15, 2001; and

U.S. patent application Ser. No. 09/855,034 entitled “High TemperatureSuper Conducting Rotor Power Leads”, filed May 15, 2001.

BACKGROUND OF THE INVENTION

The present invention relates generally to applying a super-conductingcoil to a high power density synchronous rotating machine. Moreparticularly, the present invention relates to a synchronous machinewith a conventional stator, and a magnetically saturated solid iron corerotor having a super-conducting coil.

Synchronous electrical machines having field coil windings include, butare not limited to, rotary generators, rotary motors, and linear motors.These machines generally comprise a stator and rotor that areelectromagnetically coupled. The rotor may include a multi-pole rotorcore and one or more coil windings mounted on the rotor core. The rotorcores may include a magnetically-permeable solid material, such as aniron-core rotor.

Conventional copper windings are commonly used in the rotors ofsynchronous electrical machines. However, the electrical resistance ofcopper windings (although low by conventional measures) is sufficient tocontribute to substantial heating of the rotor and to diminish the powerefficiency of the machine. Recently, super-conducting (SC) coil windingshave been developed for rotors. SC windings have effectively noresistance and are highly advantageous rotor coil windings.

Iron-core rotors saturate at an air-gap magnetic field strength of about2 Tesla. Known super-conducting rotors employ air-core designs, with noiron in the rotor, to achieve air-gap magnetic fields of 3 Tesla orhigher. These high air-gap magnetic fields yield increased powerdensities of the electrical machine, and result in significant reductionin weight and size of the machine. Air-core super-conducting rotorsrequire large amounts of super-conducting wire. The large amounts of SCwire add to the number of coils required, the complexity of the coilsupports, and the cost of the SC coil windings and rotor.

While iron core super-conducting rotors have been largely ignored byindustry, iron core rotors offer certain advantages over air-corerotors, when operated at magnetic field saturation to increase theair-gap magnetic field and power density of the machine. The advantageis that it takes considerably less super-conductor material in amagnetically saturated iron-core rotor to attain the same benefits ofhigh machine power density as compared to an air-core rotor.

High temperature SC coil field windings are formed of super-conductingmaterials that are brittle, and must be cooled to a temperature at orbelow a critical temperature, e.g., 27° K, to achieve and maintainsuper-conductivity. The SC windings may be formed of a high temperaturesuper-conducting material, such as a BSCCO(Bi_(x)Sr_(x)Ca_(x)Cu_(x)O_(x)) based conductor.

Super-conducting coils have not been adapted for commercial use in therotors of synchronous machines. Attempts have been made to incorporateSC coils into high power density generators and other such synchronousmachines. The potential benefits of adding SC coils to high powerdensity machines include light weight and compact machines. These highpower density machines typically include an air-core rotor and anair-gap stator with no stator iron teeth. However, high power densitymachines tend to be expensive and have been commercially impractical.

SC coils, their coil supports and the associated refrigeration systemshave been expensive and complex. SC coils are expensive materials, suchas BSCCO. These materials are also brittle. The coil support systemsneeded for SC coils must withstand the tremendous forces encountered inthe rotor of a large synchronous machine and protect the brittle coils.Moreover, these support systems must not transfer substantial heat intothe cryogenically cooled coils.

Further the refrigeration systems that provide cryogenic cooling fluids,such as helium, are complex and expensive. Accordingly, the cost andcomplexity of incorporating SC coils into a synchronous machine havebeen high. For SC coils to become commercially viable, their associatedcosts should be reduced to well below the advantages gained bysubstituting SC coils for conventional copper coils in the rotor.

The cost of using SC coils has become more affordable with thedevelopment of high temperature super-conducting (HTS) materials.Because they maintain super-conducting conditions (including noresistance) at relative high temperatures, e.g. 27° K, the cost to coola HTS coil is substantially reduced as compared to cooling costs forprior SC that had to be cooled to lower temperatures. There is still aneed for lower cost SC coils and coil support systems.

Super-conducting coils have been cooled by liquid helium. After passingthrough the windings of the rotor, the hot, used helium is returned asroom-temperature gaseous helium. Using liquid helium for cryogeniccooling requires continuous reliquefaction of the returned,room-temperature gaseous helium, and such reliquefaction posessignificant reliability problems and requires significant auxiliarypower.

Prior SC coil cooling techniques include cooling an epoxy-impregnated SCcoil through a solid conduction path from a cryocooler. Alternatively,cooling tubes in the rotor may convey a liquid and/or gaseous cryogen toa porous SC coil winding that is immersed in the flow of the liquidand/or gaseous cryogen. However, immersion cooling requires the entirefield winding and rotor structure to be at cryogenic temperature, as aresult no iron can be used in the rotor magnetic circuit because of thebrittle nature of iron at cryogenic temperatures.

What is needed is a HTS electrical machine that is substantially lessexpensive that prior HTS machines, and is competitively priced withexisting conventional copper coil machines. To become commerciallysuccessful, HTS machines need to become cost competitive withconventional copper machines. Potential technical areas for reducingcosts further include the coil support system, the rotor design andretrofitting existing machines with HTS rotors. Further, there is a needfor an improved rotor field winding assemblage for an electrical machinethat does not have the disadvantages of the air-core and liquid-cooledsuper-conducting field winding assemblages of, for example, knownsuper-conducting rotors.

Developing support systems for HTS coil has been a difficult challengein adapting SC coils to HTS rotors. Examples of coil support systems forHTS rotors that have previously been proposed are disclosed in U.S. Pat.Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and6,066,906. However, these coil support systems suffer various problems,such as being expensive, complex and requiring an excessive number ofcomponents. There is a long-felt need for a HTS rotor having a coilsupport system for a SC coil. The need also exists for a coil supportsystem made with low cost and easy to fabricate components.

BRIEF SUMMARY OF THE INVENTION

A high power density super-conducting machine with a rotor having a SCcoil field winding has been developed that appears to be costcompetitive with existing copper coil, low power density machines. Costsmay be reduced by employing a magnetically saturated solid core rotor, aconventional stator and a minimal coil support structure. Using thesetechnologies, an efficient HTS machine having the advantages of SC coilhas been developed. Moreover, the cost to build such a HTS machine canbe sufficiently reduced so that the machine is economical.

The HTS machine includes a conventional stator and a HTS rotor. Theconventional stator is designed for high air-gap magnetic fields thatare provided by the HTS rotor. The rotor includes a two-pole core bodyformed of a solid magnetic material, such as iron. The rotor core bodyis generally cylindrical and has flat surfaces machined longitudinallyalong its length. The HTS coil is assembled around these flat surfacesand the coil has a race-track shape that extends around the core. Therotor coil ampere-turns are sufficiently high to magnetically saturatethe rotor core and operate the machine at high air-gap magnetic fields.

The race-track coil is supported by tension coil support members thatextend through the iron core rotor body. Drive and collector shafts aremechanically fastened to the rotor core. A cylindrical shellelectromagnetic shield surrounds the HTS coil and iron core rotor body.

The iron core rotor significantly reduces the field windingampere-turns, super-conductor utilization and cost with respect toair-core rotors. The single race-track shaped HTS coil replaces typicalcomplex saddle-shaped coil windings. The tension coil support providesdirect support to the HTS coil so as to reduce the strains on the coilduring cool-down and centrifugal loading. Moreover, the coil supportsystem is at cryogenic temperatures with the coil.

The HTS rotor may be implemented in a machine originally designed toinclude a SC coil(s). The rotor and its SC coil are described in thecontext of a generator, but the HTS coil rotor and coil supportdisclosed here are also suitable for use in other synchronous machines.

In a first embodiment the invention is a high power density synchronousmachine comprising: a stator having conventional stator coils arrangedin an annulus around a vacuum cylindrical cavity; a cylindricalmagnetically saturated solid rotor core; a race-track super-conductingcoil winding extending around the rotor core, and a coil supportextending through the core and attaching to opposite long sides of thecoil winding.

In a second embodiment of the invention is a high power densitysynchronous machine having a rotate capacity of at least 100 MVAcomprising: a conventional stator having stator coils arranged in anannulus forming a vacuum rotor cavity; a cylindrical magneticallysaturated rotor core having a pair of planer sections on opposite sidesof the core and extending longitudinally along the core, and asuper-conducting coil winding extending around at least a portion of therotor core, the coil winding having a pair of side sections adjacent theplaner sections of the core.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings in conjunction with the text of thisspecification describe an embodiment of the invention.

FIG. 1 is a schematic side elevational view of a synchronous electricalmachine having a super-conducting rotor and a stator.

FIG. 2 is a perspective view of an exemplary race-track super-conductingcoil winding.

FIG. 3 is an exploded view of the components of a high temperaturesuper-conducting (HTS) rotor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an exemplary synchronous generator machine 10 having astator 12 and a rotor 14. The rotor includes field winding coils thatfit inside the cylindrical rotor vacuum cavity 16 of the stator. Thestator includes conventional stator windings 19. These windings arearranged in an annulus around the rotor vacuum cavity. The windings areseparated from each other by narrow air gaps filled with non-metallicteeth for structural support. Hence the stator is named an “air-gap”stator. Alternatively, the gaps between the stator windings may befilled with iron teeth to improve the concentration of magnetic flux inthe stator coil windings. Air-gap stators and stators with iron teethare well known in the art.

The rotor 14 fits inside the rotor vacuum cavity 16 of the stator. Asthe rotor turns within the stator, a magnetic field 18 (illustrated bydotted lines) generated by the rotor and rotor coils moves/rotatesthrough the stator and creates an electrical current in the windings ofthe stator coils. This current is output by the generator as electricalpower.

The rotor 14 has a generally longitudinally-extending axis 20 and agenerally solid rotor core 22. The solid core 22 has high magneticpermeability, and is usually made of a ferromagnetic material, such asiron. In a high power density super-conducting machine, the iron core ofthe rotor is used in a magnetically saturated state to reduce themagnetomotive force (MMF), and, thus, minimize the amount ofsuper-conducting (SC) coil wire needed for the coil winding. Forexample, the solid iron-rotor core may be magnetically saturated atmagnetic field strength of about 2 Tesla or higher.

The rotor 14 supports at least one longitudinally-extending, race-trackshaped, high-temperature super-conducting (HTS) coil winding 34 (SeeFIG. 2). A coil support system is disclosed here for a single race-trackSC coil winding. The coil support system may be adapted for coilconfigurations other than a single race-track coil mounted on a solidrotor core, such as a multiple race-track coil configuration.

The rotor core is supported by end shafts attached to the core. Therotor includes a collector end shaft 24 and a drive end shaft 30 thatare supported by bearings 25. The end shafts may be coupled to externaldevices. The collector end shaft 24 includes collector rings 78 thatprovide an external electrical connection to the SC coil. The collectorend shaft also has a cryogen transfer coupling 26 to a source ofcryogenic cooling fluid used to cool the SC coil windings in the rotor.The cryogen transfer coupling 26 includes a stationary segment coupledto a source of cryogen cooling fluid and a rotating segment whichprovides cooling fluid to the HTS coil. The drive end shaft 30 of therotor may be driven by a power turbine via power coupling 32.

FIG. 2 shows an exemplary HTS race-track field coil winding 34. The SCfield winding coils 34 of the rotor includes a high temperaturesuper-conducting (SC) coil 36. Each SC coil includes a high temperaturesuper-conducting conductor, such as a BSCCO(Bi_(x)Sr_(x)Ca_(x)Cu_(x)O_(x)) conductor wires laminated in a solidepoxy impregnated winding composite. For example, a series of BSCCO 2223wires may be laminated, bonded together and wound into a solid epoxyimpregnated coil.

SC wire is brittle and easy to be damaged. The SC coil is typicallylayer wound SC tape that is epoxy impregnated. The SC tape is wrapped ina precision coil form to attain close dimensional tolerances. The tapeis wound around in a helix to form the race-track SC coil 36.

The dimensions of the race-track coil are dependent on the dimensions ofthe rotor core. Generally, each race-track SC coil encircles themagnetic poles at opposite ends of the rotor core, and is parallel tothe rotor axis. The coil windings are continuous around the race-track.The SC coils form a resistance free electrical current path around therotor core and between the magnetic poles of the core. The coil haselectrical contacts 114 that electrically connect the coil to thecollector 78.

Fluid passages 38 for cryogenic cooling fluid are included in the coilwinding 34. These passages may extend around an outside edge of the SCcoil 36. The passageways provide cryogenic cooling fluid to the coil andremove heat from the coil. The cooling fluid maintains the lowtemperatures, e.g., 27° K, in the SC coil winding needed to promotesuper-conducting conditions, including the absence of electricalresistance in the coil. The cooling passages have an input and outputfluid ports 112 at one end of the rotor core. These fluid (gas) ports112 connect the cooling passages 38 on the SC coil to the cryogentransfer coupling 26.

Each HTS race-track coil winding 34 has a pair of generally straightside portions 40 parallel to a rotor axis 20, and a pair of end portions54 that are perpendicular to the rotor axis. The side portions of thecoil are subjected to the greatest centrifugal stresses. Accordingly,the side portions are supported by a coil support system that counteractthe centrifugal forces that act on the coil.

FIG. 3 shows an exploded view of a rotor core 22 and coil support systemfor a high temperature super-conducting coil. The support systemincludes tension rods 42 connected to U-shaped coil housings at oppositeends of each rod. The coil housings hold and support the side portions40 of the coil winding 34 in the rotor. While one tension rod and coilhousing is shown in FIG. 3, the coil support system will generallyinclude a series of tension rods with housings at the ends of each rod.The tension rods and coil housings prevent damage to the coil windingduring rotor operation, support the coil winding with respect tocentrifugal and other forces, and provide a protective shield for thecoil winding.

The principal loading of the HTS coil winding 34 in an iron core rotoris from centrifugal acceleration during rotor rotation. An effectivecoil structural support is needed to counteract the centrifugal forces.The coil support is needed especially along the side sections 40 of thecoil that experience the most centrifugal acceleration. To support theside sections of the coil, the tension rods 42 span between the sectionsof the coil and attach to the coil housings 44 that grasp opposite sidesections of the coil. The tension rods extend through conduits 46, e.g.,apertures, in the rotor core so that the rods may span between sidesections of the same coil or between adjacent coils. FIG. 3 shows therod 42 extending beyond the coil solely for illustrative purposes. Inpractice, the rod does not extend beyond the coil, but rather abuts asurface of the coil facing the core.

The conduits 46 are generally cylindrical passages in the rotor corehaving a straight axis. The diameter of the conduits is substantiallyconstant, except at their ends near the recessed surfaces of the rotor.At their ends, the conduits may expand to a larger diameter toaccommodate a non-conducting cylindrical insulator tube 52 that providesa slidable bearing surface and thermal isolation between the rotor coreand the tension rod. A lock-nut 84 holds the tube in the conduit 46.

The axes of the conduits 46 are generally in a plane defined by therace-track coil. In addition, the axes of the conduits are perpendicularto the side sections of the coil to which are connected the tension rodsthat extends through the conduits. Moreover, the conduits are orthogonalto and intersect the rotor axis, in the embodiment shown here. Thenumber of conduits and the location of the conduits will depend on thelocation of the HTS coils and the number of coil housings needed tosupport the side sections of the coils.

The tension rods support the coil especially well with respect tocentrifugal forces as the rods extend substantially radially between thesides of the coil winding. Each tension rod is a shaft with continuityalong the longitudinal direction of the rod and in the plane of therace-track coil. The longitudinal continuity of the tension rodsprovides lateral stiffness to the coils which provides rotor dynamicsbenefits. Moreover, the lateral stiffness permits integrating the coilsupport with the coils so that the coil can be assembled with the coilsupport prior to final rotor assembly. Pre-assembly of the coil and coilsupport reduces production cycle, improves coil support quality, andreduces coil assembly variations. The race-track coil is supported by anarray of tension members that span the long sides of the coil. Thetension rod coil support members are pre-assembled to coil.

The HTS coil winding and structural support components are at cryogenictemperature. In contrast, the rotor core is at ambient “hot”temperature. The coil supports are potential sources of thermalconduction that would allow heat to reach the HTS coils from the rotorcore. The rotor becomes hot during operation. As the coils are to beheld in super-cooled conditions, heat conduction into the coils is to beavoided. The rods extend through apertures, e.g., conduits, in the rotorbut are not in contact with the rotor. This lack of contact avoids theconduction of heat from the rotor to the tension rods and coils.

To reduce the heat leaking away from the coil, the coil support isminimized to reduce the thermal conduction through support from heatsources such as the rotor core. There are generally two categories ofsupport for super-conducting winding: (i) “warm” supports and (ii)“cold” supports. In a warm support, the supporting structures arethermally isolated from the cooled SC windings. With warm supports, mostof the mechanical load of a super-conducting (SC) coil is supported bystructural members spanning from cold to warm members.

In a cold support system, the support system is at or near the coldcryogenic temperature of the SC coils. In cold supports, most of themechanical load of a SC coil is supported by structural members whichare at or near a cryogenic temperature. The exemplary coil supportsystem disclosed here is a cold support in that the tension rods andassociated housings that couple the tension rods to the SC coil windingsare maintained at or near a cryogenic temperature. Because thesupporting members are cold, these members are thermally isolated, e.g.,by the non-contact conduits through the rotor core, from other “hot”components of the rotor.

An individual support member consists of a tension rod 42 (which may bea bar and a pair of bolts at either end of the bar), a pair of coilhousings 44, and a dowel pin 80 that connects each housing to an end ofthe tension rod. Each coil housing 44 is a U-shaped bracket having legsthat connect to a tension rod and a channel to receive the coil winding34. The U-shaped housing allows for the precise and convenient assemblyof the support system for the coil. A series of coil housings may bepositioned end-to-end along the side of the coil winding. The coilhousings collectively distribute the forces that act on the coil, e.g.,centrifugal forces, over substantially the entire side sections 40 ofeach coil.

The coil housings 44 prevent the side sections 40 of the coils fromexcessive flexing and bending due to centrifugal forces. The coilsupports do not restrict the coils from longitudinal thermal expansionand contraction that occur during normal start/stop operation of the gasturbine. In particular, thermal expansion is primarily directed alongthe length of the side sections. Thus, the side sections of the coilslide slightly longitudinally with respect to the channel housing andtension rods.

The U-shaped housings are formed of a light, high strength material thatis ductile at cryogenic temperatures. Typical materials for coilhousings are aluminum, Inconel, or titanium alloys, which arenon-magnetic. The shape of the U-shaped housing may be optimized for lowweight and strength.

The dowel pin 80 extends through apertures in the coil housing andtension rod. The dowel may be hollow for low weight. Locking nuts (notshown) are threaded or attached at the ends of the dowel pin to securethe housing and prevent the sides of the housing from spreading apartunder load. The dowel pin can be made of high strength Inconel ortitanium alloys. The tension rods are made with larger diameter endsthat are machined with two flat surfaces 86 at their ends.

The width of these flat surfaces fit the U-shaped housing and coilwidth. The flat ends 86 of the tension rods abut an inside surface ofthe HTS coils 34, when the rod, coil and housing are assembled together.This assembly reduces the stress concentration at the hole in thetension rod that receives the dowel.

The coil support system of tension rods 42 and coil housings 44 for thelong sides 40 of the coil, and a pair of split-clamps 58 for the coilends may be assembled with the HTS coil windings 34 as both are mountedon the rotor core 22. The tension rods, channel housings and clampprovide a fairly rigid structure for supporting the coil windings andholding the coil windings in place with respect to the rotor core.

Each tension rod 42 extends through the rotor core, and may extendorthogonally through the axis 20 of the rotor. Conduits 46 through therotor core provide a passage through which extend the tension rods. Theconduits 46 extend perpendicularly through the rotor axis and aresymmetrically arranged along the length of the core. The number ofconduits 46 and tension rods 42, and their arrangement on the rotor coreand with respect to each other is a matter of design choice. Thediameter of the conduits is sufficiently large to avoid having the hotrotor walls of the conduits be in contact with the cold tension rods.The avoidance of contact improves the thermal isolation between thetension rods and the rotor core.

To receive the coil winding, the rotor core has recessed surfaces 48,such as flat or triangular regions or slots. These surfaces 48 areformed in the curved surface 50 of the cylindrical core and extendinglongitudinally across the rotor core. The coil winding 34 is mounted onthe rotor adjacent the recessed areas 48. The coils generally extendlongitudinally along an outer surface of the recessed area and aroundthe ends of the rotor core. The recessed surfaces 48 of the rotor corereceive the coil winding. The shape of the recessed area conforms to thecoil winding. For example, if the coil winding has a saddle-shape orsome other shape, the recess(es) in the rotor core would be configuredto receive the shape of the winding.

The recessed surfaces 48 receive the coil winding such that theouter-surface of the coil winding extend to substantially an envelopedefined by the rotation of the rotor. The outer curved surfaces 50 ofthe rotor core when rotated define a cylindrical envelope. This rotationenvelope of the rotor has substantially the same diameter as the rotorcavity 16 (see FIG. 1) in the stator.

The gap between the rotor envelope and stator cavity 16 is arelatively-small clearance, as required for forced flow ventilationcooling of the stator only, since the rotor requires no ventilationcooling. The magnetic field in the gap between the rotor and the statorcouples electromagnetically the rotor coil windings with the statorwindings and directly impacts the power density of the machine.

The power density of the machine 10 can be increased by driving the ironcore rotor to magnetic saturation with higher rotor coil magnetomotiveforce (MMF). For example, a HTS rotor of just 65 inches in length hasbeen designed for a 100 MVA rated generator using rotor coil MMF of314,000 ampere-turns, whereas the same power level generator required aconventional copper rotor having a length of 128 inches and coil MMF of204,000 ampere-turns. Moreover, the 50% reduction in the length of themachine results in 35% reduction in machine size.

The end sections 54 of the coil winding 34 are adjacent opposite ends 56of the rotor core. A split-clamp 58 holds each of the end sections ofthe coil windings in the rotor. The split clamp at each coil end 54includes a pair of opposite plates 60 between which is sandwiched thecoil winding 34. The surface of the clamp plates includes channels toreceive the coil winding and connections 112, 114 to the winding.

The split clamp 58 may be formed of a non-magnetic material, such asaluminum or Inconel alloys. The same or similar non-magnetic materialsmay be used to form the tension rods, channel housings and otherportions of the coil support system. The coil support system ispreferably non-magnetic so as to preserve ductility at cryogenictemperatures, since ferromagnetic materials become brittle attemperatures below the Curie transition temperature and cannot be usedas load carrying structures.

The split clamp 58 is surrounded by, but is not in contact with collar62. The end shafts 24, 30 include a collar 62 that connects to an end ofthe rotor core 22. The collar is a thick disk of non-magnetic material,such as stainless steel, the same as or similar to the material thatforms the rotor end shafts. The collar has a slot 64 orthogonal to therotor axis and sufficiently wide to receive and clear the split clamp58. The hot side-walls 66 of the slot collar are spaced apart from thecold split clamp so they do not come in contact with each other.

The collar 62 may include a recessed disk area 68 (which is bisected bythe slot 64) to receive a raised disk region 70 of the rotor core (seeopposite side of rotor core for raised disk region to be inserted inopposite collar). The insertion of the raised disk region on the end 56of the rotor core into the recessed disk 68 provides support to therotor core in the collar, and assists in aligning the rotor core andcollars. In addition, the collar may have a circular array of bolt holes72 extending longitudinally through the collar and around the rim of thecollar. These bolt holes correspond to matching threaded bolt holes 74that extend partially through the rotor core. Threaded bolts 75 extendthrough these longitudinal bolt holes 72, 74 and secure the collars tothe rotor core.

The rotor core may be encased in a metallic cylindrical shield (notshown) that protects the super-conducting coil winding 34 from eddycurrents and other electrical currents that surround the rotor andprovides the vacuum envelope as required to maintain hard vacuum aroundthe cryogenic components of the rotor. The cylindrical shield may beformed of a highly conductive material, such as a copper alloy oraluminum. The SC coil winding 34 is maintained in a vacuum. The vacuummay be formed by the shield which may include a stainless steelcylindrical layer that forms a vacuum vessel around the coil and rotorcore.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover allembodiments within the spirit of the appended claims.

What is claimed is:
 1. A synchronous machine comprising: a stator havingstator coils arranged in an annulus around a vacuum cylindrical cavity;a cylindrical magnetic solid rotor core having a core axis; asuper-conducting coil winding mounted on the rotor core and havingopposite long side sections parallel to the core axis, wherein the coilwinding is thermally isolated from the core, and a coil supportextending through the core and attaching to the opposite long sides ofthe coil winding.
 2. A synchronous machine as in claim 1 wherein thestator is a conventional stator, and said solid rotor core ismagnetically saturated.
 3. A synchronous machine as in claim 1 whereinthe stator is an air-gap stator, and said solid rotor core ismagnetically saturated.
 4. A synchronous machine as in claim 1 furthercomprising rotor end shafts axially attached to said rotor core.
 5. Asynchronous machine as in claim 4 wherein the end shafts are anon-magnetic metal.
 6. A rotor as in claim 5 wherein the end shafts arestainless steel.
 7. A rotor as in claim 1 wherein one of said end shaftsis a collector end shaft having collector rings and a cryogenic fluidcoupling.
 8. A synchronous machine as in claim 1 wherein the rotor coreis a solid magnetic iron forging and is maintained at or above anambient temperature.
 9. A synchronous machine as in claim 1 wherein theair gap between the stator coil and the rotor is minimally sufficientfor air cooling of the stator.
 10. A synchronous machine having a rotatecapacity of at least 100 MVA comprising: a stator having stator coilsarranged in an annulus forming a vacuum rotor cavity; a cylindricalsolid rotor core having a pair of planer sections on opposite sides ofthe core and extending longitudinally along the core, wherein said coreis thermally isolated from the coil winding, and the super-conductingcoil winding extending around at least a portion of the rotor core, saidcoil winding having a pair of side sections adjacent said planersections of the core.
 11. A synchronous machine as in claim 10 furthercomprising: a first end shaft extending axially from a first end of therotor core, and a second end shaft extending axially from a second endof the rotor core.
 12. A synchronous machine as in claim 11 wherein theend shafts are a non-magnetic metal.
 13. A synchronous machine as inclaim 11 wherein the end shafts are stainless steel.
 14. A synchronousmachine as in claim 10 wherein the rotor core is a solid magnetic ironforging.
 15. A synchronous machine as in claim 10 wherein the coil has arace-track shape.
 16. A synchronous machine as in claim 11 wherein oneof said end shafts is a collector end shaft having collector rings and acryogenic fluid coupling.
 17. A synchronous machine as in claim 10wherein the rotor core is magnetically saturated and at a temperature atleast as hot as an ambient temperature.
 18. A synchronous machine as inclaim 10 wherein the coil has a coil support attached to each of saidpair of side sections and said coil support extending through the corefrom one of said pair of planer sections to the other of said pair ofplaner sections.
 19. A synchronous machine as in claim 10 furthercomprising a conductive shield around the rotor core and coil.